Epigenetic chromatin remodeling is a central mechanism for the regulation of gene expression. Pharmacological modulation of epigenetic change represents a new mode of therapeutic interventions for cancer and inflammation. Emerging evidence suggests that such epigenetic modulations may also provide therapeutic means for obesity, as well as metabolic, cardiovascular, neurodegenerative, psychiatric and infectious diseases.
The eukaryotic genome is organized into a basic packaging unit called a nucleosome, which is comprised of approximately 147 base pairs of double-stranded DNA helix wound around a histone octamer, which, in turn, consists of two subunits each of H2A, H2B, H3, and H4 proteins. Nucleosomes are further packaged into chromatin structures, which can exist in a relatively loose state of euchromatin or in a tightly packed heterochromatin structure. Transition from heterochromatin to euchromatin allows transcription of genes, although not all of the genes in euchromatin structure are transcribed. This transition from heterochromatin to euchromatin is controlled by post-translational modifications of histone proteins, including acetylation of lysine residues in H3/H4 proteins. Histone acetylation is catalyzed by histone acetyltransferases (HATs), resulting in open euchromatin structures that allow transcription of genes including tumor suppressor genes. Conversely, histone deacetylation leads to suppression of such genes and this activity is catalyzed by histone deacetylases (HDACs). Inhibition of histone deacetylases is a mode of cancer treatment and vorinostat (Zolinza®), a histone deacetylase inhibitor, has been shown to be an effective drug for cutaneous T-cell lymphoma in humans.
Histone acetylation also is monitored by bromodomain-containing proteins. Bromodomains are approximately 110 amino acid-long evolutionary conserved modules that bind to acetyllysine residues of acetylated proteins and are present in a number of chromatin-associated proteins including HATs. Bromodomains were first identified as a motif in Drosophila Brahma from which the name was derived but are also found in proteins in humans and yeast either as single-copy or contiguously repeated domains, and are thought to confer specificity for the complex pattern of epigenetic modifications known as the histone code (Cell. 1992 Feb. 7; 68(3):561-72; J Biomol Screen. 2011 December; 16(10):1170-85). The human genome encodes approximately 50 bromodomain-containing proteins (Bioinformatics. 2004 Jun. 12; 20(9):1416-27), some of which may be involved in etiology of cancer, inflammation, obesity, metabolic, cardiovascular, neurodegenerative, psychiatric and infectious diseases (Med Chem Commun. 2012 Jan. 4 3(2):123-134; Curr Opin Drug Discov Devel. 2009 September; 12(5):659-65; Discov Med. 2010 December; 10(55):489-99; FEBS Lett. 2010 Aug. 4; 584(15):3260-8; J Virol. 2006 September; 80(18):8909-19; J Virol. 2005 July; 79(14):8920-32; Curr Opin Pharmacol. 2008 February; 8(1):57-64). Thus, inhibition and/or modulation of bromodomain-containing proteins may present a new mode of pharmacological intervention for such diseases. For example, inhibition of bromodomain and extra-terminal domain (BET) family of proteins, which play a key role in controlling cell fate and cell cycle progression by recruiting transcriptional regulators to specific genomic locations (Front Biosci. 2001 Aug. 1; 6:D1008-18; J Biol Chem. 2007 May 4; 282(18):13141-5), is of particular interest as a treatment for cancer. Inhibition of the BET family of proteins was shown to be effective in rodent models for human NUT midline carcinoma, multiple myeloma, Burkitt's lymphoma and acute myeloid leukemia by indirectly reducing the expression of a proto-oncogene MYC (Nature. 2010 Dec. 23; 468(7327):1067-73; Cell. 2011 Sep. 16; 146(6):904-1; Proc Natl Acad Sci USA. 2011 Oct. 4; 108(40):16669-74). Bromodomain-containing proteins bind to acetyllysine residues of proteins including acetylated histones as well as acetylated non-histone proteins, such as transcription factors and the HIV-1 Tat protein.
Of approximately 50 bromodomain-containing proteins encoded by human genome, BET proteins represent a small protein family that includes BRD2, BRD3, BRD4 and BRDT and contains two tandem bromodomains and an extraterminal domain (J Biol Chem. 2007 May 4; 282(18):13141-5). BET proteins bind to acetylated nucleosomes and are thought to function by opening chromatin structure and/or by facilitating transcriptional initiation (Front Biosci. 2001 Aug. 1; 6:D1008-18). Inhibition of BET proteins was shown to be an effective mode of intervention in rodent models of human NUT midline carcinoma, multiple myeloma, Burkitt's lymphoma and acute myeloid leukemia by suppressing the expression of MYC gene (Nature. 2010 Dec. 23; 468(7327):1067-73; Cell. 2011 Sep. 16; 146(6):904-1; Proc Natl Acad Sci USA. 2011 Oct. 4; 108(40): 16669-74), as well as MYCN gene (Cancer Discov. 2013 March: 3(3) 308-23). MYC and homologous genes are some of the most overexpressed genes in human cancers; however, there has not been a pharmaceutical compound that directly antagonizes the activity of proteins encoded by the genes to date partly due to the lack of effective drug binding sites.
Thus, there exists a need for a means of indirect suppression of the expression of the MYC and homologous genes by inhibiting bromodomains of BET proteins which provide an effective mode of treatment for various diseases and conditions, including various cancers.